1. Field of the Invention
The present invention relates generally to a method and an apparatus for use in marine seismic exploration and more particularly to a method and apparatus for deployment of a marine seismic cable.
2. Description of Related Art
In the field of seismic exploration, it is important to determine the configuration of the rock strata underlying the earth's surface to locate subsurface structures favorable to the existence of oil and gas. In marine seismic surveying, this is accomplished by generating acoustic pulses or shock waves by sound sources such as air guns and by monitoring the resultant acoustic waves which reflect off the subsea structures by sensors such as hydrophones. In operation, the seismic sound sources and the hydrophones are towed in designated patterns behind a seismic vessel.
As the vessel moves over the area being surveyed, the seismic sound sources are activated. The sensors detect the reflected acoustic waves and provide optical and/or electrical signals (information) representative of such acoustic waves. The signals from the sensors are relayed to apparatus aboard the vessel. The signals from the sensors are processed to display maps which show the thickness and orientation of the various strata underlying the sea bed.
Commonly, an array of hydrophones is configured as a cable having a flexible jacket typically made from polyurethene or similar material. This configuration is called a streamer cable. To optimally develop marine surveys covering a wide area, multiple streamer cables are deployed in a pattern parallel to the centerline of the vessel. The streamer cables are separated from each other by desired distances to cover the marine surveying area. To obtain the required lateral distance between adjacent streamer cables, the streamer cables are attached at predetermined tow points on the cable to devices referred in the art of seismic exploration as pullavanes or paravanes. The pullavanes are towed to the side of the vessel.
Armored, optical-electrical towing cables, referred to in the art as leadins, are used to couple the streamer cables to the apparatus on the vessel. The leadins are designed to carry axial tension loads. In conventional towing arrangements, a tow line is connected directly to the leadin and to a device, such as a pullavane, which as noted above is positioned to the side of the vessel. The leadin forms a relatively large-radius loop behind the vessel (or from a boom extended sideways from the vessel) to the tow line and then a relatively small or tight loop in the opposite direction to the point where the leadin connects to the streamer cable. Such an arrangement or configuration positions the streamer cable at a desired lateral distance from the vessel. As the cable is towed through the water during surveying operations, relatively large non-axial bending forces are applied to the leadin on both sides of the point where the tow line is attached to the leadin. Erratic cross currents also tend to subject the leadin to additional non-axial bending forces.
The strength and thus the operating life of a leadin decreases greatly when a non-axial force such as a bending force is applied on the cable. This problem is exaggerated in three-dimensional seismic surveys where streamer cables are towed at larger lateral distances from the towing vessel compared to the traditional two-dimensional seismic surveying. Such larger lateral distances result in larger non-axial, bending loads on the leadins. Therefore, the operating life of a leadin may be greatly extended by reducing the non-axial forces on the leadin during operation. A common solution has been to reinforce the area around the tow point by adding additional armor to the leadin so that the leadin bends over a relatively large radius of curvature. The larger the radius of curvature, the smaller the bending loads on the leadin.
Adding additional armor to protect against these non-axial loads increases the diameter and weight of the leadin which results in higher manufacturing and operating costs. Moreover, because of the severe bend in wide tow configurations, the additional armor does not completely compensate for the non-axial forces.
There has been a long felt, yet unsolved need for a towing system that minimizes the non-axial forces acting on the leadin without increasing the size and weight of the leadin. The present invention provides an apparatus and method for towing streamer cables that addresses the above-noted problems associated with prior art towing arrangements.